Molecular Dynamics Simulation of the Influence of Temperature and Salt on the Dynamic Hydration Layer in a Model Polyzwitterionic Polymer PAEDAPS

Abstract

We investigate the hydration of poly(3-[2-(acrylamido) ethyldimethylammonio] propanesulfonate) over a range of temperatures in pure water and with the inclusion of 0.1 mol/L NaCl using atomistic molecular dynamics simulation. Drawing on concepts drawn from the field of glass-forming liquids, we use the Debye−Waller parameter (<u 2 >) for describing the water mobility gradient around the polybetaine backbone extending to an overall distance ≈18 Å. The water mobility in this layer is defined through the mean-square water molecule displacement at a time on the order of water's β-relaxation time. The brushlike topology of polybetaines leads to two regions in the dynamic hydration layer. The inner region of ≈10.5 Å is explored by pendant group conformational motions, and the outer region of ≈7.5 Å represents an extended layer of reduced water mobility relative to bulk water. The dynamic hydration layer extends far beyond the static hydration layer, adjacent to the polymer.

Do Specific Ion Effects on Collective Relaxation Arise from Perturbation of Hydrogen-Bonding Network Structure?

“…This methodology for estimating the ion–solvent dispersion interaction strength for Na + (kosmotropic), K + , and Cs + (chaotropic) results in values of 1.25, 1, and 0.85, respectively, in LJ units with a potential cutoff of r C = 3 σ. To represent the charge interaction between the ions, a dielectric constant of 0.771 was used to scale the charges to produce a Bjerrum length of 1.73 σ at T * = 0.75, where σ is expected to be 0.4 nm. , The characteristic time scale for this LJ system, τ, is on the order of a ps, consistent with the Debye–Waller parameter for water models used in atomistic simulations …”

“…The generality of this scaling for other models of water remains to be seen. Nonetheless, the use of this formalism illustrates the utility of the Debye–Waller parameter, where its value has been considered a measure of local mobility − ,, and its inverse a measure of local material stiffness. ,, …”

“…This linear relationship is then derived analytically in the Supporting Information, and the result is presented here. We then examined the physical nature of the ion hydration layer of kosmotropic and chaotropic ions with this CG model by drawing on analyses of the Debye–Waller parameter ⟨ u 2 ⟩, utilized in polymer glass-forming liquids to quantify mobility gradients around inorganic nanoparticles and more recently in studies of solvated systems. − Following previous researchers, we define ⟨ u 2 ⟩ as the mean-squared displacement (MSD) of the particles on a characteristic caging time scale, t C , a time scale on the order of a ps in molecular fluids. The value of ⟨ u 2 ⟩ has been directly related to the incoherent structural relaxation time in simulation studies, , also demonstrated in this work.…”

“…The value of ⟨ u 2 ⟩ has been directly related to the incoherent structural relaxation time in simulation studies, , also demonstrated in this work. However, ⟨ u 2 ⟩ can also be analyzed on an individual particle basis to analyze dynamic heterogeneity in solvated systems, − which shows significant differences between chaotropic and kosmotropic ion hydration. Given that the dynamic hydration layers are extended around the ions, there must come a point with increasing concentration where the hydration layers overlap, and we find that the first hydration layers in this CG model percolate at relatively low concentration, so the extended influence of the interaction energy between salt and solvent to propagate at the concentrations in question.…”

All-atom molecular dynamics simulations of polymer and polyelectrolyte brushes

“…107 This forcefield (for the ions) is also parameterized with the SPC/E water model and is used for a vast variety of polymeric systems that are modeled using the OPLS-AA forcefield. 108–110…”